It is generally known that moldings having a compact surface can be produced from polyurethane foams by in-mold foaming (e.g. German Auslegeschrift No. 1,196,864), by introducing into a mold, a reactive and foamable mixture based on compounds containing several reactive hydrogen atoms and polyisocyanates. Suitable compounds containing reactive hydrogen atoms include polyethers and polyesters containing hydroxyl groups. Suitable polyisocyanates include 2,4- and 2,6-tolylene diisocyanates, their isomer mixtures and the polyphenyl polymethylene polyisocyanates obtained by condensing aniline with formaldehyde, followed by phosgenation. Water and/or fluorochlorinated hydrocarbons may be used as blowing agents. Catalysts of the type commonly used in the production of polyurethane foams are also generally used. Providing the components are suitably selected, it is possible to produce both elastic and rigid foams.
Polyurethane foams having a compact outer skin, so-called integral skin foams, have been produced on a commercial scale for a long time (Kunststoffe 60 [1970], No. 1, pages 3 to 7).
Heavy stressed moldings can be produced from linear or slightly branched starting materials which give materials having a range of properties resembling that of elastomers. Moldings of this kind are used, for example, in the automobile industry.
The starting materials are preferably processed by the so-called reaction injection-molding technique (RIM-technique). This is a filling process, in which the highly active, liquid starting components are quickly introduced into the mold through high-output, high-pressure metering units after admixture in positively controlled mixing heads. Moldings weighing 6 to 10 kg are produced in 2 to 4 minutes, depending on wall thickness. Mixtures of substantially linear polyhydroxyl compounds of high molecular weight, 1,4-butane diol as a chain extender, and blowing agents are normally reacted with diisocyanates or polyisocyanates to produce moldings of this kind. To be useful, the elasticity modulus of these elastomers should be substantially unaffected by temperature. Thus, elastomers should be sufficiently stiff under heat, but still flexible when cold.
It was subsequently found that this property was best obtained by using ethylene glycol as chain extender. Unfortunately, materials produced in this way have the disadvantage of a totally inadequate surface hardness on removal from the mold (inadequate "green strength"). This inadequate green strength is reflected in the appearance of cracks in the surface of the molding on bending and in separation of the surface layer, even at mold temperatures of 50.degree. C. In addition, it was found that the flash around the sealing surfaces of a mold is extremely brittle, does not form a coherent film and, hence, complicates cleaning of the mold. Although these properties can be slightly improved by raising the mold temperature, this measure has a negative effect upon the stiffness of the molding when it is removed from the mold. Accordingly, it is virtually impossible to produce moldings by the RIM process which satisfy all practical requirements on a commercial scale even in cases where ethylene glycol is used as a chain extender.